Cryogenic circuit



Aug. 21, 1962 J. L. ANDERSON 3,

' CRYOGENIC CIRCUIT Filed Feb. 3, 1958 2 Sheets-Sheet 1 FIG. lb.

FIG. la. $5355 1 INVENTOR. JOHN L. ANDERSON A TTORNEYS.

1962 J. ANDERSON 3,050,683

CRYOGENIC CIRCUIT Filed Feb. 3, 1958 2 Sheets-Sheet 2 li m CUR RENTMEASURING CIRCUITRY INVENTOR. JOHN L. ANDERSON A TTORNEYS.

3,050,683 CRYOGENIC CIRCUIT John L. Anderson, Poughkeepsie, N.Y.,assignor to International Business Machines Corporation, New York, N.Y.,a corporation of New York Filed Feb. 3, 1958, Ser. No. 713,009 2 Claims.(U. 324-117) The present invention, generally, relates to cryogeniccircuitry and more particularly to a new and improved cryogenicmeasuring circuit.

It is an object of this invention to provide a new and improvedcryogenic circuit for reading out information from a superconductingcircuit.

Another object of the invention is to provide cryogenic circuits formeasuring electric current flow in a superconducting circuit.

Still another object of the invention is to provide a new and improvedcircuit arrangement adaptable for reading out information without theintroduction of electrical resistance.

A further object of the invention is to provide a cryogenic circuit formeasuring electric current fiow from one or more current sources.

A still further object of the invention is to provide a new and improvedoperative combination of a measuring circuit and a circuit for selectingone or more electric currents to be measured.

It is also an object of the invention to provide a new and improvedcommutator circuit.

Another object of the invention is to provide a commutator circuituniquely adapted for use with a cryogenic electric current measuringcircuit.

Still another object of the invention is to provide a new and improvedcommutator circuit arrangement wherein each of a plurality of currentsto be measured is provided with a current path of zero resistance.

The above and other objects and advantages are ac complished by a uniquecircuit arrangement whereby an electric current is caused to flow in awinding positioned to control the resistance of a magneticfield-sensitive circuit component. The intensity of this magnetic fieldis adjusted to a predetermined value such that the electrical resistanceof the field-sensitive component is at some value between two extremes.Then, by magnetically coupling a current the value of which is to bemeasured to the current mentioned above, its influence can be measuredwithout reflecting resistance into the circuit of the measured current.

Such a circuit arrangement has several significant uses, some of whichare illustrated in the drawings. One of the more important uses is inthe field of superconductivity although the invention is not limited tothat use.

All of the above and other objects are accomplished according to theinvention by means of such circuit structure and relative arrangement ofcomponent parts thereof as will be more fully understood from a perusalof the description below which sets forth various specific features ofan illustrative embodiment thereof.

In the drawings:

FIGURE 1a shows diagrammatically a circuit arrangement embodying theprinciples of the invention;

FIGURE 1b is a chart illustrating the relationship between the gateresistance and the gate current;

FIGURE 2 shows diagrammatically another circuit arraugement embodyingthe principles of the invention;

FIGURE 3 shows diagrammatically one circuit arrangement for utilizingthe principles of the circuit shown in FIGURE la;

FIGURE 4 shows diagrammatically a further circuit nited tates Patent Qarrangement embodying the principles of the invention; and

FIGURE 5 shows diagrammatically a commutator circuit for providing acurrent to be measured from selected sources.

Referring now to a circuit illustrating the principles of the invention,in FIGURE 1a the letter G represents a superconductive gate element,known as a cryotron. Connected in series with the cryotron gate G is anadjustable voltage source and an ammeter I. Also connected in serieswith the cryotron gate G is a winding C which is positioned in magneticfield applying relation with the cryotron gate itself, so that thecurrent flowing through the gate develops a magnetic field which, inturn, influences the resistance of the gate. A current i (t), the valueof which is to be measured, is coupled to the gate G by means of awinding C With this arrangement, the magnetic field developed by thecurrent to be measured influences the flow of current in the separatecircuit which includes the gate G.

The coils C and C have a known magnetic eifect on the gate G, whichelfect may be predetermined, for example, by controlling the number ofturns in each winding and/ or the spacing between each of the turns. Inthis 25 manner the portion of the total magnetic eifect contributed byeach coil is predetermined. For the purposes of the following detaileddescription of the operation of the circuit shown in FIGURE 1a, it willbe assumed that the coil C has the same magnetic eflfect on the gate Gas the coil C Initially, the magnitude of the voltage source is adjusteduntil the gate G is balanced somewhere on its transition characteristicbetween Zero resistance and its normal resistance, as indicated inFIGURE 1b. Then, an input current i (t) will either raise or lower theresistance of the gate G depending upon the manner in which the windingC is positioned in relation with the gate G If the resistance of thegate G is lowered, then the gate will draw more current from the voltagesource until the effect of i (t) is cancelled such that the change incurrent read on the meter I is a function of the i (t) current.

Contrary to the above, if the current i (t) causes the resistance of thegate G to increase, then the gate G draws less current from the voltagesource and the change in the current through the gate is read again bythe meter 1 as a function of i (t), similarly as above. Since thepurpose of this circuit is to measure the value of the current i (t)without reflecting or introducing any resistance into the coil C thecoil C will be superconducting as part of the circuit in which thecurrent i '(t) is flowing. However, it should be noted that it will notbe necessary for the coil C to be superconducting since its resistancewill in no way be reflected into the circuit of the coil C Also the gateG may be any magnetic field-sensitive resistor as, for example, abismuth wire.

By a slight modification in the above arrangement, the circuit shown inFIGURE 2 is formed which operates on the same principles as those justdescribed. In FIG- URE 2. a separate series branch includes a source ofvoltage V a resistor R and a gate 'G. A current I from a source V havinga resistance R, divides between the coils or windings L and L (assume inthis instance that L =L The current I which flows in the winding Ldrives the gate G to some point on its resistance transition curvebetween zero resistance and its normal resistance. The resistance of thegate G due to the current I and the current flowing because of thevoltage V determines the value of the output voltage out- A transformerT couples the current i (t) into the circuit loop formed by the coils orwindings L and L The dots shown in FIGURE 2 on the primary winding a. Cof the transformer T and also on the secondary winding L indicate thatthese ends of the transformer windings are positive at the same time andnegative at the same time. Thus a current i (t) flowing into the dottedline of the primary winding C causes a current to flow out of the dottedend of the secondary winding L which secondary current opposes thecurrent 1 and, in turn, causes an increase in the current 1 When thecurrent I increases, the magnetic effect of the winding L increases thegate resistance, resulting in an increased voltage e Conversely, adecrease in the current to be measured i (t) will cause a decrease ofthe current I and, hence, a decrease of the voltage e Again, so that noresistance is either introduced or reflected into the circuit of thecurrent i (z), all of the windings are superconducting so that noresistive component of impedance is developed. However, it may be seenthat the gate G can be any magnetic field-sensitive resistance (such as,for eXample, a bismuth wire) having, at a low temperature and in thepresence of a magnetic held, a resistance-field strength characteristicwhich provides a transition between superconductive and normalresistance values for the gate in response to variation over a limitedrange of the strength of the field.

This circuit shown in FIGURE 2 will give a wider range range of ameasurable voltage e than is obtainable with the current measurements inthe circuit shown in FIGURE 1a. This is because the current supplied tothe gate G, in FIGURE 2, by source V through resistance R will produceno voltage e when the gate G is superconducting, and will produce themaximum readable voltage e when the gate is switched to its normalresistance due to a full magnetic effect of the coil L Thus, over therange from Zero resistance to the normal resistance of the gate, acomplete range of voltage e may be obtained. For this arrangement, thecoil C must be superconducting as in the above-described circuitarrangements.

It should be noted further that, though the current i (t) is transformercoupled to the measuring circuit (FIGURE 2), a continuous outputindicative of the value of i (t) is obtained even when this current isat a steady value. Stated another way, it may be said that DOtransformer action is obtained, and the circuit is responsive to produceoutputs indicative of any variation in the current i (t) from zerofrequency to the upper limiting frequency of the transformer T. Thistype of operation results because the secondary coil L of thetransformer T is connected with coil L in a closed, completelysuperconductive loop. The net flux threading this loop may not bechanged as long as it remains entirely superconductive and, therefore,currents are produced in the loop which are dependent not upon the rateof change of current i (t) in the primary coil but upon the value ofthis current. Further, this current in the loop continues to flow oncethe current i (t) is established in coil C and varies in magnitude asthe current i (t) varies so that there is always a loop current, thevalue of which is a measure of the value of the current i (t).

In FIGURE 3 of the drawings it is shown how the principle of theinvention as described in connection with FIGURE la may be employed withan indicating circuit similar to a conventional Wheatstone Bridge. Inthis indicating circuit the voltage e will be equal to I R I R If theresistor R is made variable, then the circuit can be balanced so that 1,equals I If the value of the resistance R is selected to be equal to theresistance R and the current I is adjusted to be equal to the current Ithen the voltage e will be equal to zero.

Assume, now, the voltage V, in FIGURE 3, develops a current which isdivided into parallel path currents I and I The current I is adjusted bythe variable resistor R such that the magnetic eifect of the winding Cpositioned in inductive relation with gate G causes the resistance ofthe gate to be at some value between zero and its normal resistance. Nowif a current to be measured i (t) is applied to the coil C and it isassumed that the coils C and C produce equal magnetic effects, then thegate G becomes more resistive, thereby reducing the current I When thecurrent 1 reduces, the current I increases and the voltage output eincreases, and the magnitude of the increase in e may be related to themagnitude of the current i (t) to be measured.

Although the voltage e is not linearly related to the current i (t), acalibration curve can be drawn so that variations in the voltage e canbe converted easily to present the magnitude of the current i,,( t).

If the variable resistor R is calibrated in current, the current i (t)to be measured can be read directly from the resistor R if e isrebalanced to Zero after the magnetic eifect of the winding C isestablished. This method of reading the value of the current i,;( t) issimilar to the above method in that the circuit is initially balancedwith the current I and the magnetic effect of the coil C on the gate Gso that the voltage e is zero. Then by applying the current i (z) to bemeasured to the coil C as before, the current I decreases and thecurrent I increases. Now if the value of the resistor R is decreased torestore the balance between the currents I and I the voltage e isreturned to zero and the value of the current z' (t) to be measured isobtained directly from the calibrated resistor R As before, if thecurrent i (t) to be measured is negative, then the gate G becomes lessresistive and the current I becomes greater than the current I giving anegative voltage e related to the current i (t) by the calibrationcurve. However, whether the current i (t) is positive or negative, itwill be measurable by this circuit arrangement. It may be seen that thecoil C and the gate G need not necessarily be superconducting materials.However, the coil C must be superconducting.

The circuit arrangement shown in FIGURE 4 shows a further use to whichthe principles of the invention are applicable. The voltage change whichis developed across the gate G in order to restore the balance at apoint between zero and its normal resistance is indicative of themagnitude of the current to be measured. As described in connection withFIGURE 2, the current I supplied by the source represented by voltage Vand resistance R divides between the parallel path currents I and IFIGURE 4, the current 1,; flowing through the gate G.

The magnetic effect of a winding C positioned in magnetic field applyingrelation with the gate G is determined by the magnitude of the current Iwhich current, in turn, is controlled by a NPN transistor. For example,when the NPN transistor is more conducting, then more current I suppliedby the voltage V having a resistance R flows through the transistor,thus decreasing the current I and decreasing a magnetic elfect of thecoil C which renders the gate G more conducting.

To control the conductance of the NPN transistor, a voltage V having aresistance R is connected to the P terminal of the transistor. Theeflect of the constant current supplied by the voltage V on the terminalP is influenced by the magnitude of the current I which, in turn, isinfluenced by the magnitude of the current I A meter M indicates thevalue of the current Ibias required by the coil C in order to hold thegate G at the balance point. To illustrate further, first assume a smallincrease in the resistance of the gate G. This causes the current I todecrease and, accordingly, the current 1,, to increase permitting theNPN transistor to conduct an increased value of current I However, asthe current I increases, the current I decreases and some of theincrease in resistance of the gate G is counteracted.

On the other hand, if a small decrease in the resistance of the gate Goccurs, then the current 1,, increases and the current 1,, decreases.The NPN transistor now conducts less current I and, hence, more currentI flows through the coil C The increase in the magnetic effect of thecoil C restores some resistance to the gate G to restore the balance.

The current i (t) to be measured is applied now to the coil C (which isassumed to be equivalent in magnetic effect to the coil C By interactionbetweenthe effects of the fields of the coils C and C the current i -(t)effectively introduces or removes resistance in the gate G. Theresulting change in the current I required to achieve a new balance isrelated to the change in the current i (t) to be measured. Hence, themeter M can be designated to read directly the true value of the currenti (t).

Of course it is known that the relationship between the resistance of acryotron and the current flowing therethrough is not linear. However, byappropriate calculation this relationship can easily be determined.

As a portion of a selected measuring circuit, it is contemplated thatprovision may be made for applying a current to be measured i,,(t) asthe resultant current from one or more selected sources. FIGURE 5illustrates one circuit arrangement for selecting the current i (t) fromone or more current sources 1;, I I Sinceit is essential that noresistance be introduced or reflected into these current sources, acurrent path of zero resistance to ground is always available to eachcurrent source 1 I I either through the superconductive gate Y to groundor through the superconductive gate X and a superconductive measuringcircuit to ground.

To illustrate this arrangement in greater detail, assume that it isdesired to measure the magnitude of the current at the current source Ionly, as shown in FIG- URE 5. A pulse of electric current applied to thewindings X X,, will produce a magnetic field to drive the associatedsuperconductive gates X X resistive. These current sources 1 I areshunted to ground through the respective superconductive gates Y Y,,.The winding X on the other hand, is not energized. Instead, the windingY receives the current pulse to drive the superconductive gate Yresistive, thereby providing a path of zero resistance to ground for thecurrent source 1 through the superconductive gate X and thesuperconductive measuring circuit. In like manner, each of the currentsources I I may be applied to the measuring circuit without interactionor cross-talk between any two currents.

Of course it is to be understood that the measuring or read-out circuitsdescribed above may be used individually or in combination with othercommutator circuits, the present commutator and read-out circuitcombination being solely for illustration purposes.

It is to be understood that the above-described circuit arrangements aresimply illustrative of the application of the principles of theinvention. Numerous other arrangements may readily be devised by thoseskilled in the art which will embody the principles of the invention andfall within the spirit and scope thereof.

I claim:

1. A cryogenic circuit comprising, a gate having at low temperature andin the presence of a magnetic field a resistance-field strengthcharacteristic which provides a progressive transition betweensuperconductive and normal resistance values for said gate in responseto variation over a limited range of the strength of said field, a firstwinding magnetically coupled with said gate and electrically in seriestherewith to develop from gate traversing current a magnetic field whichbiases said gate at a resistance value on said characteristic which isintermediate said superconductive and normal values, a sec ond windingwhich is superconductive at said temperature and which is magneticallycoupled to said gate, said second winding being responsive to inputcurrent to apply to said gate another magnetic field which varies theresistance of said gate away from said intermediate value in accordancewith said characteristic, electric circuit means excluding said gate tosupply said input current to said second winding, a resistor in serieswith said gate and first winding to form therewith the first arm of anelectrical bridge, a second resistor in series with said first arm toprovide a second bridge arm, a series branch including third and fourthresistors providing third and fourth bridge arms, respectively, saidseries branch being connected in parallel with the series branch. ofsaid first and second arms to form said bridge, a voltage sourceconnected across the junctions of said two series branches to energizesaid bridge, and means to connect a voltage indicating instrumentbetween the junction of said first and second arms and the junction ofsaid third and fourth arms.

2. A cryogenic circuit comprising, a gate having at low temperature andin the presence of a magnetic field a resistance-field strengthcharacteristic which provides a progressive transition between asuperconductive and normal resistance values for said gate in responseto variation over a limited range of the strength of said field, a firstwinding magnetically coupled with said gate and electrically in seriestherewith to develop from gatetraversing current a magnetic field whichbiases said gate at a resistance value on said characteristic which isintermediate said superconductive and normal values, a second windingwhich is superconductive at said temperature and which is magneticallycoupled to said gate, said second winding being responsive to inputcurrent to apply to said gate another magnetic field which varies theresistance of said gate away from said intermediate value in accordancewith said characteristic, electric circuit means excluding said gate tosupply said input current to said second winding, a resistor in serieswith said gate and first winding to form therewith the first arm of anelectrical bridge, a second resistor in series with said first arm toprovide a second bridge arm, a series branch including third and fourthresistors providing third and fourth bridge arms, respectively, saidseries branch being connected in parallel with the series branch of saidfirst and second arms to form said bridge, a voltage source connectedacross the junctions of said two series branches to energize saidbridge, means to connect a voltage indicating instrument between thejunction of said first and second arms and the junction of said thirdand fourth arms, a plurality of first cryotrons having respective gateswhich are all connected in said electric circuit with said secondwinding to form corresponding series combinations of which eachcombination includes said second winding and the gate of only one ofsaid first cryotrons, and a plurality of second cryotrons havingrespective gates of which each is connected in said electric circuit inparallel with a respective one of said series combinations.

References Cited in the file of this patent UNITED STATES PATENTS1,474,965 Harris Nov. 20, 1923 2,189,122 Andrews Feb. 6, 1940 2,561,612Culver July 24, 1951 2,595,373 Stewart May 6, 1952 2,647,236 Saundersonet a1. July 28, 1953 2,666,884 Ericsson Jan. 19, 1954 2,737,600 Smoot eta1. Mar. 6, 1956 2,757,296 Bichsel July 31, 1956 2,763,838 McConnellSept. 18, 1956 2,782,307 Sivers Feb. 19, 1957 2,832,897 Buck Apr. 29,1958 2,944,211 Richard's July 5, 1960 2,966,598 Mackay Dec. 27, 19602,979,668 Dunlap Apr. 11, 1961 2,980,807 Groetzinger Apr. 18, 1961 OTHERREFERENCES Publication, A Review of Superconductive Switching Circuits,"by Slade and McMahon in vol. XIII, pages 574-582, October 1957, Nat.Electronics Conference.

